@PHaRTnONu
For a simple multiplication,
You could do something like
using variables from a template, and placing the result back into a template variable.
double frst = Convert.ToDouble(project.Variables["first"].Value);
double scnd = Convert.ToDouble(project.Variables["second"].Value);
return frst * scnd;
Change the '*' to '+','-' or '/' as needed
Use a variable parser (example double.Parse) if you have decimal places.
Hope that helps